Jürgen Rühe

Some thoughts on superhydrophobic wetting

Surface roughness has a profound influence on the wetting properties of a material. This fact is especially true with respect to the wetting of superhydrophobic surfaces. As a result of a special surface structure, a drop of water brought into contact with such a material forms an almost perfect sphere and even a very slight tilting of the superhydrophobic object is sufficient to cause the drop to roll off. For the description of the behavior of drops on rough surfaces two theories, namely those of Cassie and Wenzel, are often employed. However, it is currently becoming well established that both models explain the wetting behavior more from a qualitative or practical point of view rather than giving a quantitative description. For a prediction of actual contact angles, a more complex description is required that, next to thermodynamics, takes into account the kinetics of wetting. In this article, we focus on a few key aspects of superhydrophobic wetting, namely the characterization of superhydrophobic surfaces, models for the movement of drops, transitions between the Cassie and Wenzel states, and the behavior of superhydrophobic materials under condensation.

Rapid prototyping of microfluidic chips in COC

We present a novel, cost-efficient process chain for fast tooling and small-lot replication of high-quality, multi-scale microfluidic polymer chips within less than 5 days. The fabrication chain starts with a primary master which is made by well-established cleanroom processes such as DRIE or negative SU-8 resist based surface micromachining. The formation of undercuts in the master which would complicate demolding is carefully avoided. Secondary PDMS masters or epoxy-based masters which are more suitable for common polymer replication schemes such as soft-embossing, hot-embossing or injection molding are subsequently cast from the primary masters. The polymer replica are mainly made of COC and show excellent fidelity with the conventionally micromachined master while displaying no degeneration, even after more than 200 cycles. The use of other polymers such as PMMA is also possible. The process chain further includes surface modification techniques for overall, long-term stable hydrophilic coatings and for local hydrophobic patches as well as a durable sealing based on thermal bonding.

Nanopore-Based Single-Molecule Mass Spectrometry on a Lipid Membrane Microarray

We report on parallel high-resolution electrical single-molecule analysis on a chip-based nanopore microarray. Lipid bilayers of <20 μm diameter containing single alpha-hemolysin pores were formed on arrays of subpicoliter cavities containing individual microelectrodes (microelectrode cavity array, MECA), and ion conductance-based single molecule mass spectrometry was performed on mixtures of poly(ethylene glycol) molecules of different length. We thereby demonstrate the function of the MECA device as a chip-based platform for array-format nanopore recordings with a resolution at least equal to that of established single microbilayer supports. We conclude that devices based on MECAs may enable more widespread analytical use of nanopores by providing the high throughput and ease of operation of a high-density array format while maintaining or exceeding the precision of state-of-the-art microbilayer recordings.

Synthesis and swelling behavior of a weak polyacid brush

We describe the synthesis and the swelling behavior of a weak polyacid brush attached to a solid surface. Monolayers of poly methacrylic acid are generated by using self-assembled monolayers of an azo initiator and radical chain polymerization of methacrylic acid monomer at the surface of a planar substrate in situ. The thickness of the resulting surface-attached polyelectrolyte brush can be adjusted between 5 and 400 nm in the dry, collapsed state. The swelling behavior in water as a function of pH and salt concentration is investigated by multiple-angle null-ellipsometry. At high salt concentrations the brush thickness decreases due to electrostatic screening. At low concentration, however, an increase of thickness with ion concentration is found.

Mimicking the stenocara beetle--dewetting of drops from a patterned superhydrophobic surface.

This paper describes the preparation of superhydrophobic surfaces that have been selectively patterned with circular hydrophilic domains. These materials mimicked the back of the stenocara beetle and collected drops of water if exposed to mist or fog. Under the effect of gravity, the drops dewetted from the hydrophilic regions once a critical volume had been reached. The surface energy in the hydrophilic regions was carefully controlled and assumed various values, allowing us to study the behavior of drops as a function of the superhydrophobic/hydrophilic contrast. We have investigated the development of drops and quantitatively analyzed the critical volumes as a function of several parameters.

Controlled Growth of PMMA Brushes on Silicon Surfaces at Room Temperature

The atom-transfer radical polymerization of methyl methacrylate at room temperature starting from monolayers of initiators, which are covalently anchored to silicon substrates, is described. The control of layer thickness and molecular weight, a constant graft density, and a narrow molecular weight distribution of the polymers produced provide evidence for a controlled surface-initiated polymerization, and dense surface-attached polymer brushes on the SiO 2 surfaces are obtained under mild reaction conditions.

Polymer Brushes via ATRP: Role of Activator and Deactivator in the Surface‐Initiated ATRP of Styrene on Planar Substrates

The role of activator and deactivator species in the surface-initiated atom-transfer radical polymerization of styrene using CuBr/CuBr 2 /pentamethyl-diethylenetriamine as a model system is described. The influence of initially added deactivator with respect to the degree of controlling the layer growth and thickness is studieed. Variation of the activator concentration results in changes of the kinetics as wells as brush thicknesses consistent with the well-known rate laws of ATRP. System used for the generation of polymer brushes via ATRP.

The structural background of charge-carrier motion in conducting polymers

The conductivity of poly(3,4-cycloalkylpyrrole) perchlorates and the corresponding salts of poly(3,4-cycloalkylthiophene) can be analysed in terms of a mechanism assuming random hopping of charge carriers between localized states of adjacent chain segments of different chains. The conductivity can be expressed by σ=σ0 exp (–2αR) exp (–Ea/kt) where R is correlated to the size of the substituent at the pyrrole or thiophene units. Differences in conductivity between the polypyrrole and polythiophene systems can be quantitatively assessed and attributed to differences in the value of ∂Ea/∂R, this value being larger for polythiophene derivatives; Ea is an activation energy limited to the hopping process (phonon-assisted hopping). Systems with layered structures such as salts with long n-alkylsulphonate, -sulphate or -phosphonate counterions do not show a counterion-dependent conductivity. This is explained based on a structural model in which the closest packing distance remains unchanged when the length of the alkyl chain of the counterion changes. The same model explains the behaviour of poly(3-alkyl-pyrrole) salts as well. Polypyrrole salts of polymeric counterions show a behaviour similar to the materials with cycloaliphatic rings fused to the pyrrole units. This is quantitatively assessed taking the inhomogeneous distribution of counterions into account, which arises from the conformational disorder of the polymeric counterions.

New conducting polymers from 3-alkylpyrroles

Abstract New conducting polymers have been synthesized by electropolymerization of a variety of 3-alkylpyrroles. The black, spongy films of the polymeric salts exhibit conductivities ranging from 0.03 to 10 Scm−1 after compaction. Neutral poly(3-alkylpyrroles) are soluble in some common organic solvents. The solutions are stable when kept under inert gas, but degrade rapidly when exposed to air. By chemical polymerization of 3-alkylpyrroles with iron(III)- chloride polymers are obtained, which are soluble in the oxidized, conducting state. A peak observed in small angle x-ray diffraction suggests a highly disordered layered structure.

Surfaces with Combined Microscale and Nanoscale Structures: A Route to Mechanically Stable Superhydrophobic Surfaces?

Materials with superhydrophobic properties are usually generated by covering the surfaces with hydrophobic nanoscale rough features. A major problem, however, for any practical application of such strongly water-repellent surfaces is the mechanical fragility of the nanostructures. Even moderate forces caused by touching or rubbing the surfaces are frequently strong enough to destroy the nanostructures and lead to the loss of the superhydrophobic properties. In this article, we study the mechanical stability of superhydrophobic surfaces with three different topographies: nano- and microscale features and surfaces carrying a combination of both. The surfaces are generated by silicon etching and subsequent coating with a monolayer of a fluoropolymer (PFA). We perform controlled wear tests on the different surfaces and discuss the impact of wear on the wetting properties of the different surfaces.